Method of electrodepositing gold alloys

ABSTRACT

GOLD ALLOYS OF IMPROVED MECHANICAL AND ELECTRICAL PROPERTIES ARE ELECTRODEPOSITED CATHODICALLY FROM AQUEOUS ELECTROLYTES ON CONDUCTIVE ARTICLES BY MEANS OF CYCLICALLY VARYING POTENTIALS, THE POTENTIAL IN A FIRST PERIOD OF EACH CYCLE HAVING A DURATION OF AT LEAST 0.1 SECOND BEING EQUAL TO THE DEPOSITION POTENTIAL OF THE ALLOY COMPONENTS, THE POTENTIAL IN A SECOND PERIOD OF 10**-3 TO 10**-4 SECOND BEING MUCH HIGHER THAN IN THE FIRST PERIOD, AND THE SECOND PERIOD BEING FOLLOWED BY A THIRD PERIOD OF APPROXIMATELY 10**-1 TO 10**-3 SECOND IN WHICH THERE IS NO SIGNIFICANT POTENTIAL BETWEEN THE CATHODE AND THE ELECTROLYTE.

July 31, 1973 M. DETTKE ETAL 3,749,650

Mmiion OF ELECI'IRODEIOSITING 001,1) MJLOYS Fil'ed A '1 14, 1972 pm g5as ./L

FlG.l

United States Patent Office 3,749,650 METHOD OF ELECTRODEPOSITING GOLDALLOYS Manfred Dettke, Trautenstrasse 24, 1 Berlin 31, Germany; RolfLudwig, Bulowstrasse 19-22, 1 Berlin 30, Germany; and Wolfgang Riedel,Ludwigsfelder Strasse 7, 1 Berlin 37, Germany Filed Apr. 14, 1972, Ser.No. 244,209 Claims priority, application Germany, Apr. 24, 1971, P 21 21150.7 Int. Cl. C23b /42 US. Cl. 204-44 7 Claims ABSTRACT OF THEDISCLOSURE This invention relates to the electrodeposition of goldalloys from aqueous electrolytes, and particularly to a method ofimproving the properties of electrodeposited gold alloys by modifyingthe potential applied to the electrodes during deposition.

it is known to produce gold-copper alloy deposits of improved brightnessby periodically reversing the potential applied to the electrodes. It isa serious shortcoming of this known method that the amount of metaldeposited per unit of current passing through the bath is relativelysmall since some of the metal deposited cathodically is again dissolvedwhen the object to be plated becomes the anode. It has also beenproposed to employ pulsating direct current in which pulses of astrength sufficient to deposit all constituents of the alloy alternatewith pulses of at least twice the current strength. The last-mentionedmethod does not permit the deposition of alloys having uniformcomposition. The alloys produced do not show significantly improvedhardness, wear resistance, or elongation as compared to conventionallyproduced alloys.

It is a primary object of this invention to provide a method ofelectrodepositing gold alloys of improved crystal structure resulting inbetter hardness, wear resistance, and elongation and of uniformcomposition without loss in the deposition efiiciency of the appliedcurrent.

The improved binary, ternary, and quaternary alloys of the invention aredeposited on immersed conductive objects from aqueous baths by closelycontrolled cycles of potential pulses, each cycle consisting of a pulseof high voltage and a duration of to 10- second, which is preceded by aperiod of deposition potential lasting 0.1 second or longer, and isfollowed by a period of about 10- to 10" second during which thepotential is much lower than the deposition potential and practicallyzero, the several potentials being applied to the immersed object as thecathode and to the electrolyte.

Best results have been achieved when the time integral of the highvoltage pulse in each group is smaller than 5X10 voltseconds, themagnitude of the pulse is greater by about 1 to 5 volts than thedeposition voltage, and is preferably between 2 and 8 times thatvoltage. The highvoltage pulses of the several groups may be uniform or3,749,650 Patented July 31, 1973 further modulated. The potentialchanges generally described above have been found to polarize the freeand complex metal ions in the electrolyte in such a manner as to producethe results indicated above.

The desired sequence of applied potentials is achieved by means of astep generator capable of producing stepped voltages of predeterminedduration in a cycle including, for example, ten stages differing fromeach other in duration and amplitude. In the attached drawing:

FIG. 1 is a block diagram of a suitable generator; and

FIG. 2 diagrammatically illustrates the changes in the output potentialof the generator as a function of time.

Referring initially to FIG. 1, there is seen a generator whose principalelement is a decade counter 1 equipped with a 1-of-10 decoder. Thedecoder outputs control semi-conductor switches (field effecttransistors) T to T through an adapter circuit 2, the switches beingarranged at the signal inputs of an integrator 3- and of an outputsignal amplifier 4. The switches T to T select the duration of intervalst to r which are each infinitely variable by means of potentiometers Pto P The simultaneously selected respective switches T to T select theamplitudes of the output signals during the corresponding interval, theamplitudes being continuously adjustable by means of potentiometers P toP An integrator input current I defined by the position of theassociated potentiometer P is associated with each semi-conductor switchT The integrator output voltage U s satisfy the equation that is, Idetermines the slope of U s.

When each of the switches T to T is closed, the switch T controlled bythe adapter, is to be closed for a period which is short as compared tothe time of increase of U,,s in order to discharge a capacitor C. Fromthe moment at which a switch T is closed, there elapses a time t definedby the associated potentiometer P until U s becomes equal to thethreshold potential of a comparator 5. At this moment, the comparatorfurnishes a pulse to the decade counter 6 and advances the counter byone unit, that is, the next output of the decoder is activated. Thesemi-conductor switches are operated by the adapter circuit so that theyset the duration and the output signal amplitude of the subsequentinterval. The integrator, comparator, counter, and decoder operate in aclosed loop so that a new cycle of stepped potentials starts after eachgroup of ten pulses at the counter input. The counter is readily startedin position 8 of a singlepole, double-throw switch by means of a gate 7in the input circuit of the counter and stopped in position 9.

The potential pattern so produced is seen in FIG. 2.

It is essential for the change in polarisation that the high-potentialpulses A A A act on the electrolyte for a duration of 10- to 10* secondand that the periods of a potential not significantly dilferent fromzero potential t t t extend over l0 to 10- second.

The high-voltage pulses employed according to the invention do notgenerate current variations according to Ohms law but merely causeso-called non-Faraday currents which transfer or polarize the ions andcomplexes present in the cathode film, but do not discharge ions in theelectrolyte. These unsteady potential changes are produced in the mannerindicated, the potential of the Faraday or deposition current beingfollowed briefly by a potential peak which generates a non-Faradaycurrent.

In the example of a pattern of potential changes illustrated in FIG. 2,A A and A are the deposition potentials for the desired alloycomposition, their magnitudes being merely presented by way of example.The effective deposition periods are indicated as t t and t A A and Aare the potential peaks of the non-Faraday currents which must satisfythe relationship:

The periods of the potential peaks A A A are approximately to 10* secondand are indicated at t t r The periods of zero or practically zeropotential t t t having a duration of about 10 to 10- second follow thepeak potentials.

While A A A need not be zero, they must be much smaller than A A A Theperiods of deposition potential t t t must be longer than the periods ofpractically zero potential t t t and the latter must be much longer thanthe periods of the potential peaks t t t The effective depositionperiods 2 t are to be selected so that the thickness of theelectrodeposit during each individual period should not exceed 300angstrom units. The necessary deposition periods are of the order of 0.1to about 100 seconds and thus much longer than the periods of peakpotential. Good results are generally obtained when the growth of theelectrodeposit in each deposition period is of the order of 50 angstromunits.

Each cycle of three groups, as illustrated in FIG. 2, is separated fromthe next cycle by a period of approximately 10* second in which thepotential amplitude A is of the same order as A A A and approximatelyZero. The period is provided merely to fit the pattern to the availableapparatus which includes a decade counter.

While a cycle of three groups of potential steps has been illustrated inFIG. 2, the number of groups in the cycle is not critical, and goodresults can be achieved with repeating cycles having two or four groups,each group consisting of a period of deposition voltage, a shortpotential peak, and a period of zero or practically zero voltage. Thedeposition voltages A A A, need not be constant in the mannerillustrated, but may additionally be modulated by the use of analternating current generator which superimposes a sine, triangle, orsquare wave pattern on the basic potential.

There is no current reversal during any cycle so that deposited alloy isnot again dissolved.

Gold alloy electrodeposits formed with the pattern of potentialsexemplified in FIG. 2 have been found to have improved chemical,physical, and mechanical properties due to the structure of the metal.The binary goldcopper alloys of the invention are distinguished bysuperior conductivity and low surface contact resistance. The ternarygold-copper-cadmium alloy deposits are extremely ductile even in heavycoatings and are bright, and the quaternary gold-silver-nickel-palladiumalloys have surprising wear resistance.

The electrolytes employed contain alkali metal dicyanoaurates and one ormore complex bound elements of Groups IVa, Va, Ib, III], or VIII of thePeriodic Table, or mixtures thereof.

Sources of metals to be deposited and their suitable concentrations inthe electrolytes are listed below, the concentrations being expressed inmilligram atom per liter:

The electrolytes additionally may contain the usual conductive salts andbutters such as the following whose names are followed by preferredconcentrations in mole/ liter:

potassium cyanide 0.05-1.0; potassium dihydrogen phosphate 0.1-2.0;dipotassium hydrogen phosphate 0.05- 2.0; dipotassium dihydrogendiphosphate 0.05-2.0; tetrapotassium diphosphate ODS-2.0; potassiumcarbonate 0.05-1.0.

The gold alloy deposits prepared according to the method of theinvention have been used successfully in the electronics industry forcontacts and for printed circuits, and in the jewelry trade.

The following examples further illustrate the invention:

EXAMPLE 1 An aqueous electrolyte was prepared to contain:

gold as KAu(CN) :6.0-8.0 mg. atom/liter copper as -K Cu(CN) 150-240 mg.atom/liter potassium cyanide: 60-90 millimole per liter For eachspecific run, the composition was held practically constant and thetemperature was held at :1 C. at a chosen value between 60 and C. Thegenerator was set for delivering a pattern corresponding to that of FIG.2 with the following values:

tinguished by unusually good electrical conductivity and low contactresistance. They were eminently suitable for guilding electroniccomponents and for printed circuits. They contained intermetalliccompounds not known heretofore such as Cu Au.

Typical electrodeposited copper-gold alloys of the invention differ fromconventionally deposited alloys of the same composition as follows:

Alloy Conven- 0f the tlonal invention Hardness, Viekers 16 260-340350-450 Elongation, percent 0. 5-1. 6 2. 0-3. 5 Abrasion loss (4,500double strokes), 111g- 75 Porosity, m 2. b 1. 5

The effects of process variables on the color of goldcopper alloys arewell known in the art, and the commonly accepted rules of electrolytecomposition for producing a more red or a more yellow shade aresubstantially applicable to the method of this invention. A rich goldcolor is obtained at the mid-points of the composition ranges indicatedabove, that is, at 7 mg. atom/liter gold, 195 mg. atom/liter copper, 75millimole/liter potassium cyanide and a temperature of 65:1 C.

EXAMPLE 2 gold as KAu(CN) 65-100 mg. atom/liter copper as K Cu(CN)150-300 mg. atom/liter cadmium as K Cd(CN) 0.35-0.90 mg. atom/literpotassium cyanide: 60-85 millimole/liter The generator was set for thefollowing values:

The temperature of the electrolyte was held at i-l C. between 50 and 75C. for controlling the color of the deposit which could also be variedby increasing the values of A A A up to 2.0, 1.7, and 2.5 v.respectively. Depending on the specific conditions chosen, the alloyscontained 70%-85% gold, 12%-22% copper, and 3%- 8% cadmium. They wereextremely ductile even when deposited heavily, were bright, and had anelongation of up to 15%. The Vickers hardness of some of the depositswas as high as 400 -kp./mm. The electrodeposited alloy Au75-Cu22-Cd3 hadan electrical conductivity of 6.7 10* ohmr cm.- as compared to a valueof 5.5 10- ohmcm.- for a thermally produced alloy of the samecomposition.

EXAMPLE 3 A quaternary gold silver nickel-palladium electrodeposit wasobtained from an aqueous electrolyte containing:

gold as KAu(C-N) 10-50 mg. atom per liter palladium as K Pd(ON) 5-25 mg.atom/liter silver as KAg(CN) 4.5-15.0 mg. atom/liter nickel as K Ni(CN)30-150 mg. atom/liter potassium cyanide: 50-100 millimole/ liter Thegenerator was set for the following program:

Secs.

t 1O- t 0.25 t3 10 t9 10"" The electrolyte was used within thetemperature range of '60-75 C. The deposits formed were bright to have areflectance of as compared to mirror bright silver. Their color wasclosely similar to that of silver. They had compositions within limitsof 83 %-90% gold, 7%-11% silver, 0.5%-1.0% palladium, and 2.5 %-5.0%nickel. Their wear resistance exceeded that of pure gold deposits by afactor of about 50. They showed little microstress in layers up to 8 m.and were ductile so as to make them eminently suitable for platingjewelry.

In an analogous manner, the following gold alloys were prepared:

Au-Pd-Cd-Zn Au-Cu-Pb Au-Ag-Co Au-Cd-Sb Au-Cd-As Au-Cu-Sn Au-Ag-Ni Thecomponents of the electrolytes employed and their concentrations havebeen indicated above. It will be noted that complex potassium salts werelisted throughout, but it will be understood that sodium salts may beemployed as well.

Alloys containing signi-ficant amounts of more than three minorconstituents in combination with gold are not commercially useful atthis time, and no attempt has been made to produce them according tothis invention. However, there is no reason why such multi-componentalloys should not be electrodeposited from suitable electrolytes by thepulsed current of the instant invention.

It should be understood, therefore, that the foregoing disclosurerelates only to preferred embodiments of the invention, and that it isintended to cover all changes and modifications of the examples of theinvention herein chosen for the purpose of the disclosure which do notconstitute departures from the spirit and scope of the invention setforth in the appended claims.

What is claimed is:

1. A method of electrodepositing a gold alloy on an electricallyconductive object which comprises:

(a) immersing said object in an aqueous electroplating electrolytecontaining dissolved sources of the components of said alloy; and

(b) establishing a cyclically varying potential between said object asthe cathode and said electrolyte, each cycle of varying potentialincluding:

(1) a first period of at least 0.1 second in which the depositionpotential of said sources is maintained,

(2) a second period of approximately 10 to 10 second in which apotential much higher than during said first period is maintained, and

(3) a third period of approximately 10* to 10' second in which thepotential between said object and said electrolyte is not significantlydifferent from zero, said first, second and third periods immediatelysucceeding each other.

2. A method as set forth in claim 1, wherein said components are membersof the group consisting of gold, silver, copper, zinc, cadmium, arsenic,antimony, tin, lead, cobalt, nickel, and palladium.

3. A method as set forth in claim 2, wherein said components are presentin said electrolyte as cyanide complexes when they are members of thegroup consisting of gold, silver, copper, zinc, cadmium, cobalt, nickel,and palladium, and are present at hexahydroxide complexes when membersof the group consisting of arsenic, antimony, tin, and lead.

4. A method as set forth in claim 2, wherein said alloy essentiallyconsists of a major amount of gold and of a minor amount of one to threeother members of said group.

5. A method as set forth in claim 1, wherein the integral of said muchhigher potential with respect to time is smaller than 5 10- volt-second.

6. A method as set forth in claim 1, wherein said much higher potentialis about 1 to 5 volts higher than said deposition potential.

7. A method as set forth in claim 1, wherein said components areelements of Groups IVa, Va, Ib, IIb, and VIII of the Periodic Table ofElements.

References Cited UNITED STATES PATENTS Wohlwill 204DIG. Huggins 204DIG.Chester 20443 Rockafellow 204DIG Rockafellow 204DIG.

10 GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.

20443 G, 228, DIG. 9

